Essentially, this adds GLSL to the list of backends to run.
Bug: tint:1358
Change-Id: I5075df32d935a3e08733daadbe5ac9dc1e13f03c
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/77220
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>